From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:52929) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bm0sQ-00059L-64 for qemu-devel@nongnu.org; Mon, 19 Sep 2016 11:53:03 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bm0sM-00059Z-H1 for qemu-devel@nongnu.org; Mon, 19 Sep 2016 11:53:01 -0400 Received: from mail-wm0-f41.google.com ([74.125.82.41]:37030) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bm0sM-00059N-Aa for qemu-devel@nongnu.org; Mon, 19 Sep 2016 11:52:58 -0400 Received: by mail-wm0-f41.google.com with SMTP id b130so69833239wmc.0 for ; Mon, 19 Sep 2016 08:52:58 -0700 (PDT) From: =?UTF-8?q?Alex=20Benn=C3=A9e?= Date: Mon, 19 Sep 2016 16:51:35 +0100 Message-Id: <20160919155139.28371-5-alex.bennee@linaro.org> In-Reply-To: <20160919155139.28371-1-alex.bennee@linaro.org> References: <20160919155139.28371-1-alex.bennee@linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Subject: [Qemu-devel] [RFC 4/8] seqlock: use atomic writes for the sequence List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org, pbonzini@redhat.com, cota@braap.org, stefanha@redhat.com, kwolf@redhat.com Cc: mttcg@listserver.greensocs.com, fred.konrad@greensocs.com, a.rigo@virtualopensystems.com, bobby.prani@gmail.com, nikunj@linux.vnet.ibm.com, mark.burton@greensocs.com, jan.kiszka@siemens.com, serge.fdrv@gmail.com, rth@twiddle.net, peter.maydell@linaro.org, claudio.fontana@huawei.com, =?UTF-8?q?Alex=20Benn=C3=A9e?= From: Paolo Bonzini There is a data race if the sequence is written concurrently to the read. In C11 this has undefined behavior. Use atomic_set; the read side is already using atomic_read. Reported-by: Alex Bennée Signed-off-by: Paolo Bonzini Signed-off-by: Alex Bennée --- include/qemu/seqlock.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h index 2e2be4c..8dee11d 100644 --- a/include/qemu/seqlock.h +++ b/include/qemu/seqlock.h @@ -31,7 +31,7 @@ static inline void seqlock_init(QemuSeqLock *sl) /* Lock out other writers and update the count. */ static inline void seqlock_write_begin(QemuSeqLock *sl) { - ++sl->sequence; + atomic_set(&sl->sequence, sl->sequence + 1); /* Write sequence before updating other fields. */ smp_wmb(); @@ -42,7 +42,7 @@ static inline void seqlock_write_end(QemuSeqLock *sl) /* Write other fields before finalizing sequence. */ smp_wmb(); - ++sl->sequence; + atomic_set(&sl->sequence, sl->sequence + 1); } static inline unsigned seqlock_read_begin(QemuSeqLock *sl) -- 2.9.3